Dependency Injection in ASP.NET Core

Dependency Injection (DI) is a powerful design pattern that promotes loose coupling and makes applications more maintainable and testable. ASP.NET Core has a built-in DI container that simplifies the implementation of this pattern.

What is Dependency Injection?

Instead of a component creating its own dependencies (objects it needs to function), those dependencies are "injected" into it, typically through its constructor, a property, or a method. This allows the component to be oblivious to how its dependencies are created and managed.

The ASP.NET Core DI Container

ASP.NET Core includes a lightweight, built-in DI container that handles:

Registering services.
Resolving dependencies.
Managing the lifetime of services.

Registering Services

You register services in the ConfigureServices method of your Startup.cs file (or using Minimal APIs). The DI container supports three primary lifetime modes:

Transient: A new instance of the service is created every time it's requested.
Scoped: A single instance of the service is created once per client request.
Singleton: A single instance of the service is created once and reused throughout the application's lifetime.

Here's an example of registering services:


public void ConfigureServices(IServiceCollection services)
{
    // Register a service with a singleton lifetime
    services.AddSingleton<IMyService, MyService>();

    // Register a service with a scoped lifetime
    services.AddScoped<IOtherService, OtherService>();

    // Register a service with a transient lifetime
    services.AddTransient<IAnotherService, AnotherService>();

    services.AddControllersWithViews();
}

Resolving Dependencies

Dependencies are typically resolved through constructor injection. The DI container automatically provides instances of registered services when an object is created.

Example of a controller that uses constructor injection:


public class HomeController : Controller
{
    private readonly IMyService _myService;
    private readonly IOtherService _otherService;

    public HomeController(IMyService myService, IOtherService otherService)
    {
        _myService = myService ?? throw new ArgumentNullException(nameof(myService));
        _otherService = otherService ?? throw new ArgumentNullException(nameof(otherService));
    }

    public IActionResult Index()
    {
        ViewBag.Message = _myService.GetData();
        return View();
    }
}

It's good practice to inject interfaces rather than concrete implementations to maintain loose coupling.

Service Lifetime Considerations

Choosing the correct service lifetime is crucial for performance and correctness.

Use Singleton for services that are stateless or can safely share state across all requests.
Use Scoped for services that need to maintain state within a single HTTP request (e.g., database contexts).
Use Transient for services where each use should be independent.

When a service with a shorter lifetime (e.g., transient) is injected into a service with a longer lifetime (e.g., singleton), the DI container might create a single instance of the shorter-lived service and reuse it, which can lead to unexpected behavior. Be mindful of these lifetime dependencies.

Built-in Services

ASP.NET Core provides many built-in services that you can inject, such as:

ILogger<T> for logging.
IConfiguration for accessing application configuration.
IHttpContextAccessor for accessing the current HTTP context.
DbContext (from Entity Framework Core) for database operations.

Custom DI Container

While the built-in container is powerful and sufficient for most applications, you can integrate with third-party DI containers like Autofac, Ninject, or StructureMap if you require more advanced features.